Process for freeze-refining a metal



May 3, 1966 Original Filed Oct. 27, 1958 Vacuum Line O. C. AAMOT PROCESS FOR FREEZE-REFINING A METAL 5 Sheets-Sheet 1 INVENTOR. OLAV C. AAMOT, DECEASED BY RICHARD O. AAMOT, EXECUTOR ATTORNEY May 3, 1966 o. c. AAMOT PRooEss FOR FREEzE-REFINING A METAL 5 Sheets-Sheet 2 Original Filed Oct. 27, 1958 Exp Error Of Llquld Of Head Of instantaneous So||d\ 2 8 o W. 3 2 M 2 m o O o o o o 8.62m *o E250 um IO 2O 30 40 50 60 70 80 90 Percent frozen (TBS, N0. 26|) INVENTOR.

oLAv c. AAMoT, oEcEAsED BY RICHARD o. AAMoT, ExEcuroR Fig. 5.

ATTORNE Y5 May 3, 1966 o. c. AAMor PROCESS FOR FREEZE-REFINING A METAL original Filed oct. 2v, 195s 5 Sheets-Sheet 3 25E.- zoEnoaEoo (Darn shown for k-overage 8:

k-instamaneous refer to 60% frozen only) INVENTOR.

DR .M no w T nu .r CC EE DX E TrT... Om f mA AA 0.0 D WR LA H om RV.. YB B n e z 0 r f 0 a/ Fig. 6.

ATTORNEYS May 3, 1966 Original Filed 001;. 27, 1958 Stage I Derivation of bulk de osited metal for su seauent refining.

33.3% Freeze-out of aluminum from overall liquid burden-repeated twice to amass l5 tons Stage 2 First purification of SOLID- A following re-meltm 75% Freeze out oase on total aluminum content.

Stage 3 Second urification of SOI. .ID via partially urlfied SOLID-IIB,

ollowlng re-me|t|ng. 75% Freeze-out.

Stage 4 Third purification. of SDLID-A vlo 'purified SOLID-C derived from o urifled SOLID-B 84.5 reeze-out foliowin re-melting of SOLI -0 Stage 5 Partial melting pf SOLID-D for discharge as final product and final melt out of rest of SOLID-D 5 Sheets-Sheet 4.

Rotary retort CHARGE# I5 tons raw charge each fill =45tans total (0.l7% Fe 99.65 All DEPOSITi 5 tons SOLID-A (0.024% Fel each fill l5 tons total LIQUID RESIDUE DISCHARGEDl LIQUID- A I0 ons (to market as 99.5% metall ch f'" (O24 Fel (."HARGEx I5 ions of SOLID-A (0.024% Fe) DEPOSIT H25 Ions SOLID-B (0.007% Fe) @Mmmm 3.75 tens (recycled to Stage I) (0-07 /o Fe) CHARGE= (L25 (ons SOLID-B (0.007% Fel DEPOSIT= 8.44 tons SOLID-0 (0.002% Fel (WLMI-)i 2.8i tons (recycled to Stage 11) (0.02 le Fe) GHARGE= 8.44 (ons SOLID-G (0.002% Fel DEPOSIT= 7.| Ions SOLID-D of 0.0007 /o Fe L( UID RESIDUE DISGHARGED* (.34 IOIeIS 0f (recycled to Stage IUI) 0-007 F0 DISCHARGE= About Stans of final purified aluminum of about 0.00I/o Fe and low in type B impurities DISCHARGE= About 2.l tons of lSOLID-( low tyre A metal but h| ln type B impurl ies INVENTOR OLAV C. AAMOT, DECEASED BY RICHARD 0. AAMOT, EXECUTOR ATTO R N EYS May 3, 196.6 o. c. AAMo-r o 3,249,425

PROCESS FOR FREEZE-REFINING A METAL Original Filed Oct. 27, 1958 5 Sheets-Sheet 5 SPLIT FREEZE-REFINING (loadlng step only) Per Charge =`l5 tons metal (0.25% Fe) FIRST STAGEs Ikiffzii: 251mm' soi'd A De 'i ae mes o i osi acgumulate l5 tons of High Ti, V grid 0.05% Fe de oslted Solid-A ln /1 r Liquid-A: i2.5 ions 5/6 of head (0.30% Fei SECOND STAGE= 60% freeze Solid-B Deposit 7.5 tons 0.065% Fe I/2 head -B 5tons |/5 head 5"/0 FB Successive Charging V THIRD STAGE= "'""5o% fresu=25 ,ons SolidC (lower than head) i/s of need 02 F CHARGE REQUIREMENTS 90 tons 6 charglngs to store I5 tons ln retort I l= SOLID-A 2 ohargings to store I5 tons in retort 2 SOLID-B 75 tons 2 chargings to store I5 tons in retort 5 SOLID-B 2 chargings to store I5tons in retort 4 SOLID-B 30 tons 6 charglngs to store I5 tons in retort 5 SOLID-C I5 tons fi'nal liquid 5/6 combined solids qssoririga/QOSG Fe l/6 final liquid assay/ing .l0 e Fe Overall frozen 85.34 e Overall kfactor=0.079 iNVEN-l-OR OLAV C. AAMOT, DECEASED BY RICHARD O. AAMOT, EXECUTOR Fig. 8.

ATTORNEYS United States Patent O 3,249,425 PROCESS FOR FREEZE-REFINING A METAL Olav C. Aamot, deceased, late of Lewiston, N.Y., by

Richard 0. Aamot, executor, Seattle, Wash., assigner to Joseph R. Mares, Dickinson, Tex. Continuation of application Ser. No. 769,868, Oct. 27,

1958. This application Aug. 17, 1964, Ser. No. 391,071 1S Claims. (Cl. 75-68) This is a continuation of the inventors copending application Serial No. 769,868, yfiled October 27, 1958, now abandoned; whi-ch, in turn, in a continuation-impart of my application Serial No. 626,522, filed December 6, 1956, now also abandoned which in turn is a continuationin part of applicati-on Serial No. 440,886, filed July 2, 1954 now abandoned.

The .present invention relates to a new and improved ymetallurgical process. More particularly, the invention contemplates the provision of a novel process for the purification of met-als and alloys and, specifically, Ialuminum, involving treatment of relatively impure charges q of such materials by a unique freeze-refining technique.

In the inventors patent applications, mentioned above, the inventor has described and claimed a process for the freeze-refining of various metals and alloys which involves treatment, in the liquid phase, of the metallurgical charge undergoing refining while confined within a horizontallydisposed, rotating, cylindrical retort, whereby incremental portions of the impure liquid charge are gradually transformed into a purified solid phase. The process of the copending application provides for the establishment and maintenance of intensive agitation between the liquid and solid phases lthrough control of the speed of rotation of the retort to suspend the depositing solid phase increments,

as formed, against the outer walls of the retort under action of gravitational and centrifugal forces, whereby the deposited solid phase is -caused to be passed repeatedly through the liquid phase which is ldisposed in pool-like fashion in the lower section of the retort. Heat is supplied continuously to the liquid phase throughout the refining cycle in order to Amaintain the same at a temperature at least equivalent to the melting point of the -depositing lsolid phase, and ultimate separation and recovery of the purified solid and residual liquid phases is effected by draining the impure liquid phase from the retort while the purified solid phase is continued in suspension against the walls of' the retort under action of the forces induced by the rotary motion of the retort.

The present invention is concerned with improvements in the basic freeze-refining technique of my copending application,as applied specifically to aluminum and aluminum alloys. The invention finds general application, however, in the freeze-refining of any solute-solvent system in which there exists a notice-able difference in the concentration of the solute between the solid and liquid phases at equilibrium c-onditions. By way of illustration, data may be readily ascertained by referen-ce to any of the so-called binary phase diagrams, in which one metal may be considered the solute land the other the solvent, with the solvent actually constituting the main component f the metals system. In general, the :solute metal or alloy component will either function to lower the melting point of the solvent metal component, thereby producing a lower-melting point system, or eutecticum, as, for example, in Vthe system lead-tin, or the solute component l 3,249,425 Patented May 3, 1966 Micev will raise the melting point of the solvent component, such as in the system antimony-tin, in which case no eutecticum is formed. In Iaddition to the foregoing phenomena, there exists :a third possibility, namely, that in which the respective components are mutually immiscible or not readily soluble in each other. In the latter instance, the separation of two such metals or alloy systems may be effected quite readily as, for example, in the case of the lead-iron system, in which lead may be v0btained as a relatively pure by-product in the reduction of lead-containing iron ores. Thus, inasmuch as the iron and lead are not mutually soluble in each other and d0 not mix to any appreciable extent, the heavier lead component will usually separate out in the form of an independent bottom layer within the smelting unit.

In those situations in which two liquefied metals are only partly soluble in -ea-ch other, they will usually stratify into two .separate layers in which each layer will contain a certain concentration of the-principal component of the other layer, in solution with its main component. This situation is lfound to exist in systems such as lead-zinc, wherein the liquefied zinc -layer suspended on top of the heavier liquefied lead, will norm-ally contain approximately 0.7.per-cent lead dissolved therein. While such a concentration of a contaminating metal component may be considered negligible for many purposes, it represents a formidable task to effect separation of this residual lead content from the otherwise relatively pure zinc. In ygen eral, this is customarily done by application of well known distillation techniques to produce :an extremely high-purity or premium zinc product (99.99% Zn).

Since most rnetais in their liquefied ystate exhibit at least limited solubility towards one or more other metals, it is not at all uncommon to find a prin-cipal metallic component contaminated `with a variety of impurities. For example, aluminum of relatively high purity will usually contain a plurality of contaminating metals in the form of impurities. These impurities will generally fall into one or the other of the types enumeratedbefore, namely, they will either `serve to lower the melting point `of aluminum or they will raise its normal melting point. In essence, this means that when aluminum commencesto crystallize Iin solid form from a melt of relatively impure aluminum, those metals or elements which normally lower the melting point of aluminum will enrich or become more concentrated in the vresidual melt,'whereas those metals or elements that raise `the melting point of aluminum will be concentrated within the solid or crystal fraction. Considering each such potential impurity, provided there exists a difference in concentration between the solid and liquid phases of aluminum, under equilibrium or ideal conditions with respect to that element, the present invention utilizes this difference to advantagein effecting over-all purification lof the parent metal, .aluminum.

It will be appreciated that conditions norm-ally attendant to bulk industrial refining operations cannot be expected to be as efficient `or ideal as those represented by laboratory-scale operations conducted towards the construction of a phase diagram for a corresponding two or multi-component met-als system. Accordingly, as .a convenient vmeans for evaluating the degree of perfection attained in bulk refining, one may take the ratio of lthe percent of a particular impurity contained in an average sample of the whole solidified and refined metal, to the percent of the lsame impurity contained in the residu-al .im-pure liquid fraction. This ratio is herein referred to as the average K-factor or the K-factor for the impurity. For those elements which function t-o lower the melting point of aluminum, hereafter referred to as Type A Impurities, the K-factor will be less than unity, whereas for those elements which function to raise the melting point of aluminum, hereafter referred to :as Type B Impurities, the K-factor will be greater than unity. The 'following metals .represent the most common Type A impurities Afound in normal electrolytic, or virgin-grade aluminum metal:

TYPE A IMPURYITIES Iron Silicon Copper Nickel Magnesium Calcium Sodium Gallium Boron Zinc, and Manganese whereas the. following metals represent the most common Type B impurities usually found in such aluminum:

TYPE vB I MPURITIES Vanadium Titanium Chromium Zirconium, and Molybdenum The foregoing listings are not intended to be exhaustive, nor do they represent the only impurities whichcan be removed from aluminum in accordance with the general processing techniques of my invention.

As established in greater detail hereinafter, it is immaterial whether all of the Iabove impurities -occur simultaneously in the aluminum metal to be refined, or in various lesser combinations, but it is essential only from the standpoint of the process of the invention that the concentration of any one metal must not be unduly high. In the latter event, a preliminary crude refining operation can be practiced to remove any excess of .a particular impurity, following which the freeze-refining technique of the invention can be applied to the semi-refined product.

While trace quantities of virtually all of the aforementioned impurities can be detected in different lots of normal lgrade electrolytic aluminum, the principal impurities consist of the following metals (Type A) in the approximate concentrations indicated:

Percent Iron 0.06-0.35 Silicon 0.06-O.2S Copperv 0.01-0.05 Gallium 0.01

In general, the type B impurities enumerated above, i.e., vanadium, titanium, etc., will usually be present in concentrations of the order of 0.01 percent lor less. It may be said that electrolytically-produced aluminum metal is viewed as normal grade when containing a minimum of 99.5 percent aluminum, whereas high-purity aluminum is generally viewed .as containing aluminum in a minimum concentration of 99.85 percent. The latter product is considered to bea premium grade aluminum metal and demands a higher price than the usual normal-grade.y

In accordance with present commercial practice, 4bythe application of a secondary electrolytic refining of'normal grade aluminum metal (Hoopes process), a so-called super-refined metal of 99.99 percent aluminum content can be produced, Ebut the premium charged for this product is quite substantial due to the inherent additional proc- Atained by application of so-called zone refining.

essing required for its production. In addition to the super-pure electrolytic product, it is known that `an .aluminum of 99.999 percent `aluminum content can be olIJ- Tl e latter process utilizes a molten zone traversing .a long, solid ingot of aluminum to yflush Type A impurities from the major length Iof the ingot, concentrating the same `at one end thereof and Type B impurities at the otherend. It is generally conceded, however, that there is no existing process capable o-f producing 99.99 percent pure aluminum metal at a cost competitive with that of theaforementioned Hoopes electrolytic rening technique.

The present invention contemplates the perfection .of a.

process whereby super-purepaluminum metal (99.994- percent Al) can be produ-ced from normal grades yof aluminum or even lower -grades such as scrap metal, efii- -ciently, economically, and on a tonnage scale. Of course, it will be readily understood that the process is equally applicable to the refining of. many other metals and metallic charges, and may be applied, also, to purification problems of a different nature, such as aqueous solutions, waxy emulsions, etc.

As pointed out hereinbefore, my copending application Serial No. 626,522 describes the use of a horizontally-disposed cylindrical retort which is capable `of being rotated :about its own axis, and eventually inclined 'to a vertical position for discharge of liquid fractions contained therein. In the use of thisretort for refining purposes, the interior of the sameis preheated to a temperature above the melting point of the metal to be refined, lthe unit is then charged with the metal in liquefied or molten form, and rotated under continual application of slow-cooling through the external walls. At the same time, lheat is supplied to the interior of the retort while the metal is undergoing refining. As a result, the desired purified metal is progressively deposited out of the liquid melt 'onto the surface of the internal lining of the retort, such that continual operation of the -unit in this manner is effective in building up an accumulated solid layer suspended lagainst the retort lining. It has been demonstrated by actual experimental data set yforth in the copending application that metallic tin of especially high-purity can be progressively produced through successive freeze-refining operations performed on the crystallized solid phase recovered from a preceding stage of refining of the same type. In addition, it was further demonstrated that an equivalent purification could be effected by application of this general freeze-refining technique to aluminum metal.

It is a specific object of my present invention to describe -further improvements `and discoveries relating to the general application of Ifreeze-.refining to aluminum metal. In particular, the invention contemplates the provision of an improved refining process which is .capable of effecting enhanced purification of aluminum in a more efiicient :manner than is possible in accordance with the basic processing techniques described in my original application.

The process of the present invention is based, in part, on the observation that considerable oxidation of alummum occurs during the freeze-refining operation as conducted in accordance with the teachings of my aforementionedapplication, owing t-o the relatively high speed of rotation of the retort, which I previously considered essential in order to Iachieve: good refining action. For eX- ample, an essential'feature of my original process involved the use of an interiorly-disposed gas fiame to retard the freezing or crystallization action. While the inventor has found that the use of internal heating is still a prime requisite to the obtainment of good K-factors, i.e., -good separation of impurities, the inventor haspfurther observed that there exists a substantial difference in the efficiency of the over-all refining action depending upon the nature, quantum, location `and method whereby such internal heating is applied to the charge undergoing refining. By way of illustration, the inventor has `attempted many refining Operations utilizing an internal heating source consisting of `a coaxially-p-ositioned radiant electrical heating element extending `along the entire length of the retort, but have found that comparatively poor results are obtained in all instances with this type of an arrangement, whether heat is supplied via an electric element or as a gas flame.

Quite unexpectedly, however, it has been found that substantially improved efficiencies can be obtained when the thermal energy of the internal heating source is supplied principally to the liquid pool of metal, i.e., without supplying any .appreciable heat to the remainder of the metal in the retort, other than that required to maintain the ceiling of the retort at a temperature slightly higher than the temperature of the liquid metal, i.e., to prevent undue deposition of metal to occur during the upper arc of the overall-rotational cycle. It is further lfound that this type of heating can be effected most advantageously by directing the heat towards the inner-frontal refractory cone of the retort in the form of a shielded gas fiame, such that these surfaces of the retort absorb the sensible heat of the fiame .and give -it off to the liquid metal during rotation of the retort.

Still further improved results are obtained with an arrangement providing for deflector elements on the rear wall of the retort, whereby the liquid metal can be elevated to promote a forward surge of metal along the up-going side of the retort wall, thereby effectively cross-washing the solid deposit while providing good longitudinal mixing of the liquid metal. This arrangement has been found to be extremely effective in establishing and maintaining an even temperature and composition for the liquid metal while the purified metal is being deposited in stratified manner on the retort wall, in that, the solid deposits are constantly surged through the liquid metal pool during the entire refining operation.

In addition to the foregoing observations, the inventor has found that by applying an inclination or tilt to the l retort during the refining operation, a much greater charge of liquid metal can 'be treated, and the longitudinal mixing action for the liquid metal is much superior to that obtained with the retort rotated in a horizontal plane. Under these conditions, it has been observed that in addition to a strong forward surge of metal on the upgoing side of the retort, a liquid film of metal can be made to spiral on the retort wall from the rear to the front of the retort throughout the refining operation. By way of illustration, as established by the experimental data set forth hereinafter, the tests conducted with the retort operated in a tilted position show a consi-derable improvement in the K-factor, coupled with an increased capacity which, at an angle of approximately from the horizontal, is almost double the normal retort capacity when operated by rotation in an ordinary horizontal plane.

The inventor has further found that in lieu of the simple refractory lining, a previously deposited coating of high-purity aluminum metal on the walls of the retort forms a much superior surface for promoting growth of the solid phase during the refining operation.

The combination of the foregoing features further demonstrate that tlie speed of rotation of the retort can be lowered to such an extent that the undesirable oxidation of aluminum metal during the refining cycle is greatly reduced. Additional suppression of oxidation effects can be achieved by utilizing a small amount of salt liux, as for example, a mixture of alkali chlorides, and by maintaining maximum fiuidity of the liquid metal at all times through application of the interior gaseous flame source in the manner detailed above. By the addition of suitable vacuum equipment to the retort in conjunction with use of an electric inner-frontal heating metals.

A significant criterion of successful' refining in accordance with the process of the present invention is apparent upon discharge of the residual liquid melt at the end of a particular refining cycle. Thus, when the desired quantity of solid metal has been deposited on the retort wall, the liquid residue must be discharged as a thin liowing metal,'free of crystalline growth habit, and containing a minimum of over-lying dross, leaving a hollow cylindrical solid deposit on the retort wall, having an even surface of massive fine-sized crystal habit.

In the application of the freeze-refining process of the invention to vacuum conditions, the inventor preferred to employ a retort of the double open-ended type with the rear opening being connected to the vacuum equipment for exhaust. For open refining, I have used advantageously a single open-ended retort which, as will be readily appreciated, is more amenable to rotation on an axis inclined to the normal axis of the cylin- -drical retort. In either event, however, I prefer to employ a heating element disposed just inside the front opening of the retort, and consisting of either a shielded gas heater or a short electrical radiant heating element which is inserted after the retort has been charged.

The effective limit of refining for the process of the invention with respect to those elements which lower the melting point of aluminum, corresponds to that ofthe eutectic concentration of the first element to reach this concentration, usually iron. After this concentration has been reached, further deposition of solid metal will take place under condition -of constant composition, namely, metal of the eutectic concentration with respect to iron content, whereas other elements forming eutectics only when much higher concentrations of the same are reached, such as silicon, copper and galliurn, for example, will continue to concentrate in the liquid. For example, the eutectic limit for iron in aluminum is 1.9 percent Fe; that of silicon in aluminum is 12.55 percent Si; that of copper in aluminum is about 33 percent Cu; that of nickel in aluminum is about 5.7 percent Ni; etc. Additional eutectic limits for the lvarious impurities listed hereinbefore may be obtained by reference to appropriate phase diagram texts available on this subject, insofar as they are known presently.

In accordance with the binary phase diagram for iron and aluminum, for example, it is not possible to accomplish more than a certain degree of refining for each cycle of operation, and it therefore becomes necessary for the production of super-pure aluminum to repeat the refining cycle several times. That is to say, repetition of the refining operation insures that one will obtain highly purified solid aluminum metal as one endproduct, and, for the other, an impure residual liquid metal of composition close to the eutectic limit with respect to the concentration of iron contained therein, since iron will usually be the first impurity to reach near eutectic concentration.

While the process of the present invention requires a plurality of successive refining cycles, it is to be notedthat in -each such cycle the results obtained are almost equivalent to those which should be obtained under theoretically ideal conditi-ons, i.e., conditions corresponding to those employed in a determination of the melting point curves for the phase diagrams of the correspond- In other words, the K-factor obtained by the refining technique of the invention actually approximates that of the theoretically ideal K-factor as determined from the phase diagrams of the metals involved.

It will be readily apparent that it is essential to any l large scale refining operation that the number of refining cycles be maintained at a minimum in order to render the process most economical from the standpoint of consumed energy. To this end, in the refining process of the invention, the deposited solid metal phase accumulated on the retort wall at the end of any refining cycle, is close to its melting point, and may be rendered molten by the 7 simple application of heat energy in an amount equivalent to the heat of fusion of aluminum (approximately 169 B.t.u. per pound of aluminum metal). On the other hand, the liquid residue which is discharged from the retort at the end of each refining cycle will always Vbe close to the required temperature for a subsequent freeze-refining cycle conducted to recover an additional quantity of a slightly less-purified solid phase. Accordingly, it need only be poured into a standby retort preheated to the optimum operating temperature, and sufficient heat supplied to make up radiation losses incurred during the discharge and transfer operations. In essence, therefore, it will be seen that although the process of the invention involves a series of successive refining operations, these are conducted in such manner that an over-all reduced cost of refining is achieved as compared with presently known techniques,

-It is believed that the foregoing conclusion may be best understood by a brief description of the purposes underlying the multi-stage nature of my process. Thus, at the inception of a refining operation, the concentration of impurities in the liquid phase will be at an absolute minimum, with respect to content of A-elements, and will increase as the successive cyclic refining continues. This continual concentrating of Type A impurities within the liquid phase will proceed rather slowly at first until such time that approximately half of the original liquid metal has been recovered as the deposited purified solid phase from the retort walls. After the volume of the liquid melt has reached one half of that of the original charge, it will become concentrated with respect to impurities much more rapidly. That is to say, since the concentration of any one impurity in the depositing solid phase can be viewed at all times as a constant fraction of the concentration of the same impurity contained in the liquid phase, the purest solid will be deposited at the beginning of the refining operation, with the concentration of impurities increasing relatively slowly until approximately one half of the original burden has been removed as deposited solid, and then substantially more rapidly as the liquid in equilibrium with the impurities enriches more rapidly with respect to A-impurites.

By way of illustration, the inventor has found that the composition of the liquid melt, again with respect to type- A impurities only, changes only moderately up to a point where approximately one-third of the total available aluminum metal has been frozen out of the liquid phase, and, the initial portions of the deposited solid from this fraction are not appreciably different from the last portions to be deposited out of this initial one-third of the over-all burden. At the half-deposited point, however, the concentration of A-irnpurities in the liquid phase will be almost twice as high as that of the original total liquid mass, and, in the same manner, at the three-quarter-deposited mark,fthe liquid phase may be almost four times as impure in A-elernents as was the original burden.

On the basis of the foregoing, one may employ ran initial retort operated to freeze out approximately onethird of the total available aluminum metal, thereafter passing the residual liquid4 from that retort to a second retort wherein one-third of the metal remaining in the liquid isfrozen out, and, finally, the residual liquid from the second retort is passed into athird retort in which onethird of its residual aluminum content is frozen out. A plurality of retorts can be employed in the initial refining stage to insure adequate charges, in a volume sense, for the second and third stage retorts. Operating in this manner, which I shall term split freeze-refining, one obtains on the basis of all three solid fractions, approximately 70.3 percent of the over-all aluminum content available for recovery, whereas the final liquid residue from the third stage retort will contain approximately 29.7 percent of the available aluminum. As a net result, the over-all purity of these combined solid fractions with respect to content of A-elements will be substantially greater than that obtainable if the same amount of metal, namely, 70.3 percent by weight, isfrozen out of the liquid melt in a single cycle of refining. In addition, the very first solid separated out will contain the bulkof those elements which raise the melting point of aluminum (Type B), and laterdeposited solids will have diminishing contents of these impurities. Of course, this technique may be subject to change in accordance with such factors as the degree of purification required,.and the over-all economics of any specific operation.

It is believed that the foregoing objects and features of the invention, as well as the invention itself, may be best understood by reference to the following detailed description and experimental data, taken in conjunction with the accompanying drawings, wherein:

FIG. l is an elevational section view of a typical rotary retort used in practicing the process of my invention, and illustrating the associated vacuum metallurgical techniques employed to avoid oxidation of refined aluminum metal;

FIG. 2 is a similar elevational section View of a tiltable, single open-ended rotary retort illustrating the associated gas-fired heating element and defiector mantle in preferred operating position therein;

FIG. 3 is a sectional end View of the retort illustrated in FIG. 2 taken along the line 3-3 of FIG. 2, and illustrating the refractory lifters utilized to secure more cihcient longitudinal mixing of a liquid charge contained in the retort. This View is equally illustrative of a similar section taken through the retort shown in FIG. 4 of the drawing;

FIG. 4 is an elevational section view of the type of tiltable, single open-ended rotary retort used in the experimental tests described hereinafter;

FIG. 5 is a graph illustrating an actual plot of the composition of refined solids as a function of the percentage solids frozen out of solution, and of the corresponding liquid phase in equilibrium with the depositing solid hase; p FIG. 6 is a similar graph illustrating comparative instantaneous and average values, for different K-factors from 0.1 to 0.3, for both the liquid and solid fractions,

of the concentration of impurities as a function of deposited metal;

FIG. 7 is a schematic flow sheet or flow diagram illustratingrthe multi-stage technique employed to produce super pure aluminum in accordance with my invention; and

FIG. 8 is a schematic flow diagram illustrating an example of the split freeze-refining technique of my invention as described in greater detail hereinafter.

In general, the rotary retorts used in the practice of myV present invention 'are quite similar to those which have been described in substantial detail within the inventors aforementioned copending application. With particular reference to FIG. 1 of the drawings, there is'illustrated a vacuum-type retort consisting of a substantially cylindrical outer shell 10 of steel or similar material fitted with removable end-sections 11 and 12 which are each adapted to provide air-tight closures in contact with the shell 10 through suitable mated sealing nring-s or mounting flanges 13 provided around the .outer periphery thereof. The retort is adapted to be rotated about its axis by means of the roller-idler system indicated schematically in FIG. l by reference numerals 14-'17, under action of any suitable prime mover (not shown). As also indicated in FIG. 1, by the curved arrow, the retort is adapted to be tilted forward through a arc for loperational efficiency as explained hereinafter, and f-or discharging products contained therein. Reference numerals 18 and 19 are intended to depict the respective positions-of the solid and liquid phases during rotation of the retort about its normal 4horizont-al axis. The interior-facing walls of the retort as well as the removable end-sections are lined with a suitable refractory material 2t). The right-hand end 11 9 of the retort contains an electrically heated resistance rod -21 projecting into the retort a limited distance and supplied via the slip-ring brushes 22. The opposite end of the retort is provided with a suitable connecting port 23 communicating with the interior of the retort for applying vacuum conditions to the retort during the refining cycle.

The retort illustrated in FIG. 1 is particularly suited for use in refining operations in which complete avoid- -ance of oxidation must be maintained for the aluminum metal undergoing refining. In addition, with a condenser attached to the retort in Vthe manner described in my copending application Serial No. 440,886, the unit may be employed to distill out any contaminating metals yof the type of zinc and/ or magnesium which might otherwise 1nterfere with the normal refining operation when present in excessive amounts within the raw charge material, i.e., scrap aluminum, etc. Refining of aluminum metal under vacuum also serves to remove dissolved gases, and particularly hydrogen, thereby providing for the production of substantially non-porous refined metal.

With reference to FIG. 2 of the drawings, the inventor has illustrated a single open-ended retort in which like elements are designated by the same reference numerals 'as `assigned hereinbefore with respect to FIG. 1. In the retort of FIG. 2 one end of the retort is closed by a steel plate 24 and refractory liner 25 similar to the shell and lining 1040 utilized throughout the remainder of the retort. In general, the refractory lining 25 at the retort end is somewhat heavier than that required on the cylindrical wall surfaces of the retort. The entire lining may be suspended from the retort shell by suitable hangers such as a heavy mesh steel net or the like which is in turn welded to the external shell.

In the retort of FIG. 2, the heat input required during the refining cycle is obtained by means of a gas-fired burner 26 which projects into the end of the retort and fires against a suitable deflector mantle 27 serving to expel gases of combustion out of the retort as illustrated by the small arrows in FIG. 2. Of course, the retort of FIG. 2 may also be equipped with an electrical radiant heating element similar to that illustrated in FIG. 1, the principal objective being in either case to obtain uniform heating of the inner frontal refractory surfaces of the retort as designated generally by reference numeral 28 in FIGS. 1

'and 2, and ultimately, heating of the liquid metal pool by contact with said refractory surfaces during rotation of the retort. The inner frontal conical part of the refractory 30 may be made of special heat conductive material, `such as Carborundum, to facilitate absorption of heat by the heat source as well as to conduct heat faster over the whole conical frontal part.

Both the retorts of FIGS. 1 and 2 are equipped with defiector or lifter elements 29, incorporated in the lining on the rear walls of the retorts for purposes of producing longitudinal stirring of the liquid phase with accompanying evenness of composition and heating in this phase. The arrangement of these defiectors may best be seen by reference to FIG. 3 of the drawings. In general, the deflectors consist of refractory projections formed in substantially propeller-like configuration and radiating outwardly from the center of the rear wall. In essence, these elements serve much the same function as ordinary impeller blades within a mixing device, since they are moved with respect to the liquid phase upon rotation of the retort and tend to agitate the liquids in a longitudinal direction throughout the refining cycle. The retortshown in FIG. 2 is tiltable, upwards as indicated by the reverse arrow, as well as forwards; the tilting mechanism not being shown.

In FIG. 4 there is illustrated the modified single opene'nded type of retort which was employed in the experimental tests described hereinafter. This unit is provided with a single axially-mounted drive shaft 31 which is fixed directly to the Outside of the rearwall of the retort for purposes of facilitating operation of the retort at various inclinations and rotational speeds. This experimental retort could be tilted upwards also,l as indicated by the arrows in FIG. 4, to a 12.5 angle with the horizontal. The lining of the retort is similar to that illustrated in FIGS. 1 and 2 except that 4an inner refractory lining of asbestos cement 20 is coated with an outer refractory lining 32 of Alundum `or an equivalent material. In actual practice with this retort, a Monel metal deflector 27 w-as used in combination with a lgas-fired heating source to derive the desired inner frontal -heating effects.

With reference to the flow sheet of FIG. 7, the inventor has indicated by means of schematic retorts in combination with typical charge compositions, the technique employed in producing super-pure aluminum metal via a multi-stage refining operation conducted with the progressively purer solid phases recovered from preceding stages of refining. Thus, the initial stage of the cyclic operation is utilized to produce the bulk deposited metal to be refined in the subsequent stages of the process, whereas the discharged residual liquid phase from each stage of refiningl from the second stage on may be recycled for use in similar retorts appropriately spaced in time sequence from the first retort. In essence, as may be seen by reference to the charge data shown'on the flowsheet, the intermediate deposited solid phase desig- Vnated as Solid-D (Stage 4) is substantially free of iron and related impurities (Type A) which tend to lower the melting point Iof aluminum. On the other hand, this metal will contain a relatively high concentration of Type B impurities such as titanium, vanadium, and other metals which tend to raise the melting point of aluminum, but these elements are already concentrated into the first part of the solid deposit, and by melting out the last deposited solid product of Solid-D at this point in the Icycle, this product recovered from the partial melt-up operation (Stage 5) becomes the final super-pure aluminum desired, whereas the residual solid deposit, now called Solid-E, functions as a collector for Type B impurities. It should be understood, that in some cases only two stages of purification performed on the bulk deposit from the first stage refining may be sufficient to produce aluminum metal of 99.99% purity. This depends directly on the type of raw material used for supply purposes, i.e., the quantum and nature of the impurities contained within the aluminum metal charged to Stage 1 of the cycle.

With reference to FIG. 8 of the drawings, the inventor has shown in schematic owsheet form a technique utilizing the basic principles of my invention which is hereinafter referred to as split freeze-refining. The retorts illustrated in FIG. 8 may all be viewed as being equivalent to the Stage-1, or'bulk loading retorts of the multistage refining process shown in FIG. 7, as will appear more clearly from the following description of the intended operation of this system.

All five of the retorts illustrated in FIG. 8 are of equal capacity and all have the same freeze-refining capacity on an hourly basis. Thus, assuming for -a maximum bulk charge of 15 tons, retort No. 1 in FIG. 8 will freeze-out 21/2 tons of solid in 21/2 hours, or at a rate of 1 ton per hour. This represents one-sixth of the incoming charge, or so-called head metal to that retort. The residual liquid, 121/2 tons of metal from retort No. 1 is then supplied, at the end of 21/2 hours of freeze-refining, to retort No. 2, for example, which constitutes one of a group of three retorts (Nos. 2, 3 and 4) all operated, in turn, on liquid feed from retort No. 1. These retorts will function to freeze more metal out of the liquid feed derived from retort No. 1, such that in 7%. hours a.

deposit of 71/2 tons should be accumulated in these retorts, assuming continuous operations, and the residual metal discharged from each into retort No. 5 will be 5 tons. Accordingly, each of the 2, 3 and 4 retorts effects approximately 60% freezing-out of their charges, or 1/2 of the over-all head metal forming the original charge.

molten aluminum metal. insertion of an airblast-butane gas burner.

Retorts 2, 3 and 4 are spaced on 21/2 hour lead-times with respect to each other, such that retort No. 1 is fully occupied in servicing one of these retorts each 2%. hours. Assuming, as stated above, that each retort has a storage capacity of `15 tons of deposited solid metal, following two cycles of refining on material supplied from retort 1, each of retorts 2, 3 and 4 will be fully loaded with 15 tons of metal. Whenever a retort reaches its capacity in this manner, it is then started on a refining cycle as outlined in FIG. 7 to further `refine this l5 tons of partially purified metal. That is to say, retorts 2, 3 and 4 of FIG. 8 successively become Stage 2 retorts within the system of FIG. 7 working towards the production of Solid-B metal.

Retort 'is supplied with the liquid residue, in turn, from each of retorts 2, 3 and 4, and functions to freezeout from these 5 ton increments, one-half of the available 5 tons, or, one-sixth of the original charge to retort No. 1. This retort will discharge on 2 1/ 2 hour cycles, a final impure liquid which also represents one-sixth of the original head metal (Z1/z tons). If desired, a sixth retort can be added to the system to further concentrate the liquid discharge from retort No. 5 into a still higher and more nearly eutectic liquid product. This retort could, in turn, be supplied also with the return liquids dischargedfrom the other retorts later in the cycle.

It will be seen that in sequential order, retort No. 2 will be the first to attain its capacity of l5 tons of solid deposit, `followed by retort No. 3, then Nos. 1, 4 and 5, in that or-der. At this point, i.e., with all retorts carrying tons of frozen-out solids, the following will have been accomplished:

Retort No. 1 will contain metal which is high in Type B impurities, but substantially free of Type A impurities.

Each of retorts Nos. 2, 3 and 4 will contain metal which is of uniform composition, low in Type Brimpurities, but containing Type A impurities in slightly higher concentrations than the metal in retort No. 1.

Retort No. 5 will contain metal which is free of Type B impurities, but which is almost as impure as the head metal With respect to Type A impurities. At the same time, the ultimate liquid residue recovered from retort No. 5 will be concentrated to a high degree in Type A impurities, and may be further refined to reach a near eutectic concentration with respect to iron, for example. Following this, the liquid may be subjected to a separate treatment for iron removal, and gallium concentration as described hereinafter, such that more aluminum metal may be eventually recovered from it. Other Type A impurities, and notably gallium, will be more and more concentrated in this product, along with silicon, copper, etc.

It is believed that the invention may best be understoodby consideration of the following actual applications of the foregoing principles and procedures to the refining of aluminum metal. In these operations, the inventor employed a cylindrical retort similar to that illustrated in FIG. 4. The retort shell, formed of 1/s inch steel plate, measured 141/2 outside diameter by l5 in length. It was fitted with closely-spaced small iron L-hooks mounted on the interior surface of the cylinder, and projecting inwardly about 3A", which served as anchors for an initial layer of asbestos cement of about l1/2" thickness. The frontal opening in the retort measured about 3 in diameter, and was adapted to receive small aluminum ingots for a total charge of about'twenty (20) pounds of The melting was achieved by The retort was arranged to rotate about its own axis and could oper ate in a horizontal position or in an upwardly tilted position, at angles up to 1292 degrees from the horizontal. The speed of rotation was controllable within the range -131 r.p.m., corresponding to peripheral speeds of the inner cylindrical wall within the range of from 1.0 to 5.6

12 feet per second. For discharge purposes, the retort could be tilted into a full vertical position.

In the experimental refining runs, the retort was suitably pre-heated to a red heat, and thereafter the appropriate charge of aluminum ingots was inserted and melted down to liquid metal by means of the butane gas burner. For a normal charge lof about 20 pounds of aluminum, it was found that the gas burner could reduce the metal to the liquid state in about 45 minutes. During the melt-down a small amount of oxide dross was usually formed, and this was skimmed off before the refining cycle was com'- menced. The temperature of the molten metal was thereafter adjusted fairly close to its melting point, and the retort set in rotation, such that natural cooling, principally through the-retort walls," effected freezing-out of metal adjacent the walls.

In the initial test runs, the results of which are presented in tabulated .form in Table I below, no internal heating was employed, but rather, the retort was closed to ingress of air during the actual freeze-refining by means of a thin steel cover provided with a central opening for insertion of a gas pipe adapted to introduce a protective mantle of methane or butane gas. The steel cover was tted relatively loosely over the front end of the retort, such that gas escaping from the interior of the retort could be burned exteriorly of the same.

By withdrawing the gas pipe and the steel cover, it was possible to observe the over-all refining action, effect temperature measurements, and generally determine the appropriate time for discharge ofthe residual liquid phase. It was found that when the retort was set in rotation at a speed of the order of 98 r.p.m., and adjusted to a temperature of about 20 C. above the melting point of the aluminum, a thin liquid film of aluminum ywas picked up immediately by the refractory walls, whereas the bulk of the liquid phase formed a pool in the bottom of the retort. This pool was continually agitated with a sloshing motion, principally in a transverse direction. At the up-going side of the retort, the metal pool is lifted and gradually recedes in wave-like motion, whereas at the down-going side of the retort the liquid is depressed slightly due to the drag-action of the retort wall passing through it. In essence, however, the surface of the liquid pool remains substantially horizontal. When freezing commences, the film carried on the retort wall demonstrates a diffuse reflection, rather than a mirror-surface reflection, with lines appearing in the form of liquid ridges which constantly change position in a longitudinal direction. Some liquid drips from the ceiling of the retort indicating that the liquid film has a tendency to contract into ridges, probably due to the high. surface tension of aluminum resisting the spread of a thin film over the complete surface area of the aluminum carried on the Walls. An oxide film follows and is lifted off on the metal pool and dragged across the pool, taking with it dross particles through the upper swing of the retort. After the initial solid coat of aluminum metal is deposited on-the wall of the retort, the freezing proceeds smoothly and` the level of the liquid phase begins to drop. Unless heat is supplied to the interior of the retort during this stage in the over-all refining cycle, crystals will develop in the liquid, since the temperature tends to stabilize at the natural melting point of the metal due to the intimate and extensive agitation of the solid phase through the liquid phase. In addition, it is believed that many crystals are actuallyv torn off the retort wall due yto this agitation, particularly during the initial phase of the refining cycle where the crystalline growth habit on the refractory wall is relatively weakly bonded, and the physical strength of the thin solid layer is low due to its heated condition. tals in the liquid phase will tend to multiply rapidly and increase in size, since the cooling action functions at a relatively constant rate. The only metal which will attach itself to the retort wall will then be that crystallizing from the rapidly enriching (A-impurities) liquid phase, in

Underthese circumstances, the crys- 13 that, the larger crystals formed in situ within the liquid phase are unable to be attached to the wall rmly enough to resist the impact of the washing action produced when they again strike through the liquid pool upon rotation of the retort.

By reference to the tabulated data presented in Table II below, there is shown the comparative results of a series of runs conducted with use of gas-flame heating supplied to the general interior of the retort during the freeze-refining cycle. This heat-input functions to delay freezing, thereby permitting buildup of a multilayer of extremely thin crystalline deposits on the retort wall, and to decrease the undesirable formation or accumulation of crystalline growth habit within the liquid phase. to be noted that no flux was used in conjunction with the tests tabulated in Tables I and II. The concentration of iron was studied preliminarily, since iron is generally the predominant impurity in aluminum and therefore the most important from the standpoint of control and removal.

Table I NO INTERNAL HEATIN G Freezing Time: Approximately 18-22 minutes. Speed of Rotation: Varied.

It may be concluded on the basis of the foregoing tests that the application of internal heat brought about substantially improved results and, in fact, virtually cut the K-factor in half. Thus, in the initial tests conducted Without internal heating, the liquid discharged was, without exception, a thick, crystalline, semi-liquid mass, and in the case of Test No. 4 it was discharged in the form of a dough-like ball under conditions which changed so rapid ly that just a few minutes before discharge from the retort the liquid had not been of such consistency. In the case of Tests Nos. 6-10, in which internal heat was applied, the residual liquid discharge was substantially more uid Y in all cases, although it still contained some crystalline It isv growth habit and some dross, probably due to' oxidation resulting from the presence of water vapor in the combustion gases from the burner.

In an effort to reduce oxidation effects while still maintaining the relatively high rotational speed of 98 rpm., which was initially thought to be necessary for the obtainment of good K-factors, the inventor next resorted to Weight (lbs.) Percent Fe Percent Test Speed Frozen o1 K-faetor, N o` (.r.p.m.) Total SOL/Hq.

Chg. Sol. Liq. Chg. Sol. Liq.

Table Il INTERNAL HEATING Freezing Time: -45 minutes. Speed of Rotation: 98 r.p.rn. (constant).

Weight (lbs.) Percent Fe Percent Test Speed Frozen of K-factor, N o. (rpm.) Total sol./liq.

Chg. Sol. Liq. Chg. Sol. Liq.

The above runs were conducted with wire scrap metal containing, in part, some iron cores, which accounted for certain variations in the iron content for the respective batches. The sequence of operations with respect to upgrading the original scrap was as follows:

Tests Nos. 1-6 Primary reining of crude or head metal.

Tests Nos. 7-8 Rening of solids from Tests Nos. 1 5.

Test No. 9 Rening of solids from Tests Nos'. '7.-8.

Test No. 10 Refining of mixed solids from Tests 6 and 9.

SPECTROGRAPHIC ANALYSIS Per; Per- Per- Per- Per- Per- Per- Test No. cent cent cent cent cent cent cent Si Fe Ca Mg Cu Ti V 9 (Solid) v .02 .02 X P .02 P P 10 (Solid) .03 .03 X P .02 P P X=N0t detected at .001%.

P =Present but less than .005%.

internal electric heating utilizing a resistively-heated lament contained within a protective tube of graphite which extended along substantially the entire axial length of the retort. It was considered that this form of heating would permit an accurate determination of the quantity of heat required to achieve the desired results detailed hereinbebefore. A transite disc was employed to permit closure of the frontal opening in the retort. The electric heating element was centered through the transite disc, which was in turn provided with a small opening for insertion of a thermo-element to permit regular temperature measurements'. In addition, a gas pipe was mounted in the assembly centered below the graphite tube to permit introduction of a protective rnantle of dry methane gas into the retort, with the gas being permitted to burn outside the retort during the rening operation.

In these tests, the relatively crude metal was heated and drossed initially, and rotation was then commenced with the metal maintained about 50 C. above its melting point. The electric heating element was then energized and the run continued until the desired amount of refined metal was solidified against the retort walls. In preliminary runs under these conditions, the amount of power supplied to the resistive lament was increased successively from about 800 watts up to approximately 2100 watts, which proved t0 be too much heat, with 190() watts seemingly representing optimum power input and permitting a reasonable rate of freezing out the solid deposit. On the otherhand, however, it was observed that the liquid metal still contained crystals upon discharge from the retort.

The results of these tests are presented in tabulated form in Table III below. It is to be noted that the would be to restrict the application of the heat energy to the inner-frontal portion of the refractory lining which in turn would serve to effect moreeicient and selective heating of the liquid metal since the metal is contacted directly and bodily with this portion of the refractory lin- D aluminum metal used in these tests proved to contain a ing during rotation of the retort. In theory, such an arfair amount of titanium and vanadium, which appeared, rangement would serve to reserve the cylindrical wall in particular, when the initial solid deposits were subsurface of the retort for the accommodation ofthe dejected to further refining for enhanced purity. Of course, positing solid phase.v this caused some interference with the otherwise quite 10 To the foregoing end, the retort was modified to prodependable relative method of analysis by the permangan- Vide an open end having the butane gas burner inserted nate technique. a limited distance inside this end of the retort. In addi- Table III INTERNAL ELECTRIC HEATING Speed of Rotation: 98 r.p.m. Freezing Time-varied as indicated. Head Metal: 100 lbs. 99.8% virgin aluminum.

Mins. v Percent K-factor, Test No. Freeze Chg. Sol. Liq. Chg. Sol. Liq. Frozen sol./liq.

oi Total It should be noted that Tests Nos. 17 and 18 were contion, a metallic shield Was provided at the tip of the ducted as primary refining operations, i.e., on head metal, burner to deflect the burner flame outwardly towards the whereas Test No. 22 was effected in conjunction with inner surface of the conical front end, thereby serving to metal equivalent to Solid-B of the flow sheet of FIG. 7 repelfmost of the combustion gases outwardly ythrough such that the deposited solids recovered from the latter 30 the charge opening, while at the same ytime maintaining test consisted of metal equivalent to. Solid-C of FIG. 7. this opening sufficiently heated to insure that no metal The permanganate analysis marked with an asterisk in would freeze thereon during discharge of the retort. Table III reflect the error due to the titanium and vana- In addition to the foregonig modifications, the refracdium contents of the metal undergoing analysis. The I, tory lifters (reference numeral 29, FIGS. 1, 2, 3 and 4) tabulated data presented below represent the spectro- 3 were added to the-rear refractory wall of the retort. As graphic analytic results obtained with the metal from Test explained hereinbefore, these consisted essentially of a 22 a's compared to the head metal: plurality of refractory projections cemented to the normal SPECTROGRAPHIC ANALYSIS Per- Per- Per- Per- Per- Per- Per- Per- Per- Material cent cent cent cent cent cent cent cent cent Si Fe Ca Mg Zn Mn Cu Ti V Head Metal .06 .l0 X P P P P .007 .02 No. 22 (Solid-C) .01 .0s X P P .005 P .04 .07

X=Not detected at 0.001%. P=Prcsent but. less than 0.005%.

It may be concluded from the foregoing results that lining for purposes of including continual longitudinal only slight elimination of iron was effected, whereas the mixing of the liquid phase. It was. considered that this elimination of silicon was quite good, and both titanium mixing action would insure that the liquid metal would and vanadium were considerably concentrated in the be uniformly contacted against the heated frontal por- Solid-C metal. The input of electrical heat energy, reptions of the retort even when this phase became the relaresenting approximately 1900 watts, was equivalent to tively minor part of the overall retort contents. Lastly, about 38.5 pounds of melted aluminum per hour, assumit was found that the addition of a small amount of a ing that only the heat of fusion, or 169 B.t.u. per pound solid flux prior to commencing the rotary motion of the was neededV to effect the necessarymelting. Since the retort served to minimize oxidation effects to insignifinatural freezing rate was approximately 35 pounds of cant values in terms of theoverall refining action. In metal per hour, as indicated by the results tabulated in particular, the inventor found that a flux consisting of ap- Table I, i.e., without application of internal heat, it would proximatley equimolar parts of sodium chloride and poappear that with the electrical energy supplied an average tassium chloride with approximately 5 to 10% by weight of approximately 13.5 pounds per hour was the actual of lithium chloride added thereto serves admirably for freezing rate obtained. That is to say, about 56% of the the purpose intended. This ux was generally added to supplied heat was utilized for useful purposes. the metal inan amount equivalent to approximately 20 In many other tests, again resorting to an interiorly grams after skimming off the usual oxide dross. The burning gas llame ,for additional heat, .it was confirmed flux, having a melting point well below that of aluminum that in general a K-factor of about 0.20 could be obtained metal, melts immediately and yslowly distributes itself for iron, but considerable dross was made and results were over the entire exposed surface of the liquid phase. It quite erratic at times. was subsequently found that in operations in which a more On the basic of the observations made with respect to liberal vuse of these salt fluxes was made, such as 20 the foregoing tests, as well as the general principles ungrams, initially,kfollowed by four to six additions of about derlying the freeze-refining technique of my invention, `it 10 grams each during the freezing cycle, greatly improved appeared as a theory that the uniform distribution of heat separation of Type B elements (titanium, vanadium and energy within the retort would actually serve to defeat the zirconium) wasl obtained iin theresulting solid. Apparfreeze-refining action. An entirely different principle ently, the salt flux reacts slowly with the liquid aluminum 17' thereby forming aluminum chloride which vaporizes out of the retort. At the same. time, there is nitrogen present, as verified by the dross formed during refining which invariably contains nitride of aluminum. seemingly, a.

beneficial separation -of the B-elements into the solid is hastened and improved under these conditions, in that, it is known, for example, that chlorine and nitrogen is effective in removing titanium from impure aluminum. Surprisingly enough, it was further found that the flux had a beneficial action in seemingly serving to increase the wetting and spreading action of the initial liquid metal film against the wall surface of the retort. In other instances, the vinventor also employed a commercial liux consisting of magnesium chloride, potassium chloride, calcium chloride, and a very small amount of calcium fluoride (Dow Flux No. 230) with good results.

As may be seen by reference to the progress-ive refining action illustrated in the foregoing table, the process of the invention is capable of reducing an initial iron content of 0.24% in the head metal to a residual content of 0.002% in the Solid-D product which contains about 99.98% A=l. yIn addition, most of lthe other Type A inipurities are eliminated to a similar extent, where-as the Type-B impurities are concentrated in the Solid-D metal to such an extent that they must be purged therefrom to produce ultra-pure aluminum metal. l

Following the test runs tabulated in Table IV above, which were conducted at a retort speed of 98 r.p.m., two additional tests were conducted as shown in Table V below, at a reduced retort speed of 52 r.p.m. It was found that the lower speed of rotation resulted in considerably less oxidation of the aluminum metal.

Table V Heating: Inner-frontal with deflector shield.

Metal: Composite. Speed of Rotation: 52 r.p.m.

Type A Impurities, percent Type B Impurities, Test percent No. Weight (lbs.)

Cu Fe Si Ga Ti V Zr 147 028 143 09 017 0007 0007 002 014 044 035 009 001 001 003 .057 .34 .20 003 000 .000 000 13 18 27 Indef. Indef Indef.

l50 Charge (19.25) 0017 006 007 0019 .007 .009 004 Solid. (12.75).. 000 001 002 001 010 014 006 Llquld (9.50) 005 016 018 004 .'000 000 001 K-factors Indef. 062 11 25 10 14 The combined modifications detailed above served to enhance the overall refining action substantially, and, in tests conducted under these conditions, up to 87% weight of the total liquid phase was successfully deposited upon the retort walls, with 13% by weight of the total charge being discharged at the end of the refining cycle in the form of a thin-fiowing liquid residue free of crystalline growth habit and containing only small amounts of oxide dross.

Following a series of preliminary runs to adjust to 4the modified operating conditions, controlled runs were con-A ducted towards up-grading a quantity of aluminum wire scrap, considered high-grade in terms of scrap specifications, but containing only 99,52% aluminum. A series of five preliminary refining cycles was conducted on the Y head metal to derive an initial deposited Solid-A, after .alyzed spectrographically, yielded the following results:

Table IV Heating Inner-frontal with deector shield Speed of Rotation 98 r.p.m. Head Metal 115 lbs. wire scrap aluminum.

By direct comparison of the data tabulated in Table V with those tabulated in Table IV, it may be seen that the reduced retort speed also results in substantially improved K-factors. It may be c0ncluded,'therefore, that the agitation and associated refining act-ion achieved at a retort speed of approximately 52 r.p.m. represents nearer to optimum conditions of operation, in that the lower speed seemingly enhances the separation of impurities and in particular, Type B-impunities. The freezing time for Test No. 147, in Table V w-as 58 minutes, whereas for Test No. it was 100 minutes, corresponding to freezing rates of 12.9 pounds per hour and 7.6 pounds per hour, respectively.

Quite accidentally, following conclusion of the test runs detailed above, and by reason of an attempt to offset a reduced retort capacity caused by uneven relining of the retort, it was found that further improved refining action could be obtained when the retort is operated in a position inclined to its normal rotational axis. In particular, it was observed that under these conditions, the greater depth of the pool of liquid metal accumulated in the rear of the retort effects substantially irnproved longitudinal mixing of the metal, serving to induce a spiraling liquid film of metal extending from the Type A Impurities, percent Type B Impurities,

percent Test No. and Start Mat. Weight (Lbs.)

Cu Fe Si Ga Ti V Zr 92-90 (Avg. 0f 5 runs Charge (15.17) .06 .24 12 .019 .0006 0005 0018 from headmetal). Solid-A (8.42) O29 080 050 010 001 001 003 Liq. A (6.75) 1l 47 22 031 000 000 000 K-factors 26 17 23 32 Indef. Indef. Indef.

98-100 (Avg-from Solid-B (13.75) 013 126 022 005 002 002 004 Solid-A 101-102 (A)vg.-from Solid-C (12.50) 004 007 009 003 002 003 003 Soli 103 (From SOlid-C) Solid-D (13.75) 001 002 004 002 002 003 006 Note-Return Liquids not shown,

"19 rear to thefront of the retort. Apart from the superior separatory action obtained with the retort operated in this manner, thef'capacity of the retort is greatly increased, to the extent that at a tilt ofapproximately 121/2 from the horizontal, the overall capacity is almost double that of the normal capacity with the retort operating in a horizontal position. Additional runs were conducted on the bas-is of this innovation utilizing head metal consisting of 200 pounds of individual 6-lb. ingots of so-called normal electrolytic aluminum. The results of these runs are presented in tabulated form in Tables VI andVII below, wherein all of the analyses were effected 4by spectrograph techniques:

- By reference to the foregoing analytical data, it will be seen that Solid-C is almost free of Type A-impurities, but is relatively high in Type B impurities, and-this is also the case for the Solid-D product derived from the original liquid fractions, but to a somewhat lesser extent, in that, the primary separation resulted in the absorption of most of these elements into the resulting solid product produced. In both eases, therefore, it is the iinal liquid which will actually constitute the purest aluminum.

The ultimate residual liquid fraction recovered from the operations summarized in Table VII, and further concentration, was -found to be quite impure with respect to iron content, assaying 0.81% Fe. Thisproduct was sub- Table V1 [Speed of Rotation: 66 r.p.m. Iuelined from Horizontal-4] Weight (lbs.) Percent Fe Percent Test No. and Frozen K-factor, Start Mat. of sol./liq.

Chge Solid Liq Chge. Solid Liq. total 2.12 (Head Metal) 20. 43 12. 68 7. 75 143 043 30 61. 8 143 216 (Solid-A) 20. 93 10. 75 10. 18 057 011 105 5l. 3 105 217- 21. 68 13. 06 8.62 014 003 030 6l). 2 100 Average 116 Table /II g {Speed of Rotation; 65 r.p.m. Inclined from Horizontal-7.5"]

' Weight (lbs.) Percent Fc Percent Test No. and Frozen K-faetor, Start Mat. of sol./liq.

' Ohge. Solid Liq. Chge. Solid Liq. total 226 (Liquid-A) 24. 06 15.37 8 69 .1GO 043 41 63. 8 .106 y227 (Liquid-B). 24. 50 14. 31 10 19 042 008 084 58. 4 O95 228 (Liquid-C)- 25. 00 15. 25 9 75 007 002 019 61. 0 105 Average 102 In Table VI the tests include typical examples of primary, secondary and tertiary refining oper-ations conducted with the original head metal, representing the` `solids from solids method. In addition to these tests, a fourth refining operation was conducted with the Solid- B from Test No. 217 of Table VI, and the results (Solid- C) of this run are presented in tabulated form in Table VIII below. The various residual liquid fractions recovered from the tests of Table VI were also subjected to` refining, grouped together by analyses to give about the same analyses in the respective charges as were shown for the tests in Table VI. The results of these operations are presented in tabulated form in Table VII above,

-and this method may be termed Solid from Liquids refining. In this series of liquid reiinings, a fourth (Solid- D) fraction was made with a corresponding liquid phase,

andthe analytical data for these products are also presented in Table VIII below.

jected to further refining for purposes of extracting a purer aluminum metal therefrom, and, at the same time, to ascertain the degree of concentration of impurities that could be obtainedin the corresponding liquid fraction. In these operations the relatively highly impure liquid was employedA as the starting material to derive a still more contaminated liquid, while retaining .the deposited solids Table VIII Total Cu Fe Si Zn Ni Ga Ti V Al Contained, percent Solid-C 000 000 002 000 000 000 045 033 99. 913 Liquid-Q 002 004 009 001 000 002 O02 005 99. 970

K-factors 22 22. 5 6.6

Solid-D 000 001 002 001 000 000 008 009 99. 973 Liquid-D 002 003 008 002 000 003 001 000 99. 977

K-fnetors 33 25 50 8. 0 9. 0

Original Head Metal sample- 0185 165 .,160 000 002 014 0035 004 99. 63

Noter-Total Al content based on 19 elements as impurities, hereinafter.

21 equivalent to the Solid-D of Table VIII. The tabulated dat-a presented in Table IX below represent the analytical results obtained lfor the rst two stages of 'this operation as well as the -last two stages:

Table IX Heating: Gas Burner with Metal Deflector. Speed of Rotation: 66 r.p.m.

the metal increased. [For example, when drilling the Rgtgrt Tilted 4 from Horizontal for Tests Nos. 240 and 241 and 12.5 forv Tests Nos. 244 and Weight (lbs.) Percent Fe Percent Test frozen K-factor, No. of total sol./1iq.

Charge Solid Liquid Charge Solid Liquid 240 19. 44 l1. 44 8.00 81 35 1. 46 58. 8 .24() 241.--- 20. 69 14.19 6. 50 .31 l2 .72 68. 5 157 244 27. 50 20. 50 7.00 007 002 022 74. 5 .091 245- 19. 88 14. 38 5. 50 002 001 O05 72. 3 200 ADDITIONAL ANALYSES Sample Cu Fe Si Zn Ni Ga Mg Ti V Al 240 Head 082 81 55 012 008 048 038 .000 000 99. 448 245 Solid 000 O01 002 001 000 000 000 001 001 99. 989 245 Liquid 000 005 017 002 000 001 000 O 000 99. 97

K-factors 12 50 It will be noted by reference to Table IX that the tina-l solid product of this series actually represents the purest metal, since the star-ting material was a liquid fraction Which had previously undergone several reductions in its Type B impurites content, and was, therefore, practically free of the elements ltitanium and Vanadium. The resulting super pure solid product (No. 24S-Solid) was again subjected to freeze refining and distributed into a corresponding liquid and solid phase, and it was found that a still further reduction of the iron content and silicon content and silicon content in the deposited solid took place. The tests listed indicate, therefore, that the method is inherently capable of effecting a f-ar reaching purification. The spectrographic analytical ydata for the two fractions obtained in the foregoing operation are tabulated below.

ANALYTICAL DATA samples with an electric drill, it was observed that the initial solid Was easy to penetrate and produced relatively short yturnings, whereas the final solid was -dilicult .to drill and produced long uniform turnings. IIn a similar manner, the liquid products gradually changed during the'suc- -cessive stages of refining, from a brittle metal in the first test producing small chips when drilled, to a ductile meta-l for the last test producing larger and more flexible chips. Lastly, the color of the metal was extremely silvery and shiny at the higher purity, and Iabove 99.9% purity the :brilliancy of the metal is extremely enhanced as contrasted, for example, with metal of 99.63% Al which exhibits a `diffuse white color rather than true brilliancy.

The following additional tabulated data (Table X) were collected in further refining operations conducted with the retort operated at an inclination of 12.5 `from Test No. 246

Sample Cn Fe Si Zn! Ni Ga Ti V Zr TotalAl,

percent 246 so1id .000 .000 .001 .002 .000 .000 .002 .002 .002 99.985 246 Liquid -.001 001 .004 .003 .000 .001 .000 .000 .00o 99.924

K-factor .25 .75 2.0 2.0 2.0

1 Zinc by contamination.

In later tests, where more solid metal was lavailable the horizontal. The metal used consisted of a composite corresponding in analysis Ito No. 2'46, a still further reproduct derived from the preceding tests, and the retort fning brought the purity of the liquid fraction up to 99.995 Was equipped with the frontal butane gas burner having percent aluminum. a monel metal dellector functioning to deflect the gas In the foregoing test, commencing with a charge of 65 llame against the inner frontal refractory wall and to 13.75 pounds, a solid phase of 7.94 pounds was deposited expel thegases of combustion out of the front opening in during 25 minutes freezing time, or at a rate of about 19.2 the retort. The tests were conducted for comparative pounds per hour. It is of interest to note that in this purposes at two dilferent rotational speeds, namely, 86 series, the appearance of the successive solid phases as rpm. for Test No. 249 and `66 r.p.m. for Test No. 251. scollected on the cylinder wall changed markedly. Thus, 70 About 25 grams of salt ilux was used` in each run. The the initial solid (Test No. 240) has a coarse weave patcharge Weight for Test No 249 was 27.56 pounds, with tern which became increasingly ner in texture as the purithe Weight 0f the deposited Solid PhaSe, frOZeIl in abOllt ty of the metal increased, until the final solid (Test No. 70 minutes, being equal to 18.5 pounds, representing a 245), which had an almost mirror-like surface. Accordfreezing rate of about 1'6 pounds per hour. The charge ingly, it may be possible to estimate the relative purity of weight for Test No. 251 was 22.12 pounds, with the weight 3,249,425 23 of the deposited solid phase being 11.69 pounds, frozen in about 47 minutes or at a freezing rate of about 15 pounds per hour.

24 accompanies a slightly reduced temperature for the liquid phase, is advantageous during freezing-out of the solid phase, since the depositing solid phase would be in equi- Table X Test No. Cu Fe Si Zn Ni Ga Ti V Zr Cr Total Al,

percent 249 (Charge) 100%.- 005 022 037 003 002 007 006 004 002 U01 99. 908 249 (Solid) 67.4%. 002 O 002 002 000 003 O09 005 002 002 99. 954 249 (Liquid) 32.6%- .011 057 .O88 005 006 015 002 99. 816 K-faetor 182' 088 1. 36 40 Indef. 20 9. 0 6. 0 1. 0 2. 0

251 (Charge) 100% 001 002 007 0015 0005 001 007 007 0035 0015 99. 968 251 (Solid) 52.8%. 000 001 002 O01 000 000 013 013 .004 002 99. 961 251 (Liquid) 47.2% O02 004 012 002 A001 002 001 001 001 001 99. 969

K-factor Indef. 25 167 50 Indef. Indef. 13. 0 18. 0 4. 0 2.0

These tests, at lreasonably high freezing rates, show good K-factors, it being again confirmed that best K- Y factors for iron, silicon and gallium are obtained at a medium purity range, and also that the Type B-impurities are best eliminated at lower rotational speeds.

In `the higher purity range of Test No. 251, it may be noted that the distributions of the elements iron and silicon still represent a useful K-factor, indicating that the eXtreme limit of potential purity was not reached. The accuracy of even the spectrograph is limited at this point.

In an effort to demonstrate the attainable concentration of impurities in the liquid phase, three different runs Table Xl Speed of Rotation: 66 rpm. Retort Inclined: 7 from horizontal.

librium with the liquid phase only `at one certain temperature, above which the liquid would redissolve deposited solids. Ideally, therefore, the temperature of the liquid phase should be lpermitted to adjust itself consistent with the phase diagram, while still supplying suflicient heat such that crystalline growth habit present therein will -be dissolved. On the other hand,vit is clear from the experimental data presented that the process is not overly sensitive to use of temperatures slightly above the theoretically ideal temperature, and, in point of fact, for very close separation of a highly puried solid it may be advantageous to operate at slightly higher temperature values, thereby effecting increased freezing time but improved K-factors.

In order to demonstrate the advantages of the split Freeze Weight (lbs.) Fe, Percent Percent K-iactor, Test Time Frozen sol./liq. No. (Mins.) l of total for Fe Chge. Solid Liq. Chge. Solid Liq.

The respective freezing rates for the foregoing tests were: 12.7 pounds per hourV for Test No. 225; 12.75 pounds per hour for Test No. 240; and 7.85 pounds per hour for Test No. 247. The melting point of the liquid in Test No. 247 was determined to be about 651 C., or about 9.2 C. below that of pure aluminum metal. In

v contrast thereto, the melting point of the solid from Table VIII, was determined to be about `662 C., or almost 2 above the melting point of pure aluminum. Apparently, the titanium and vanadium contents of the latter metal are responsible for the increased melting point.

, It was concluded on Athe basis of the foregoing results that a limit of 1.50% iron may be reached in the liquid phase while still maintaining a useful K-factor, although the K-factor will become less effective for separation purposes the closer one gets to the eutecticum composition between iron and `aluminum (1.9% Fe; M.P.=654 C.). It is apparent that an increased rate of freezing, which obtained in the rst stage was not as high as the average purity of the metal which could be produced by freezingout a greater percentage of solids. In addition, it took Alonger to freeze the reduced portions, of solid than was to posited solids were subjected to a differential melt-out,

such that the outer layers of the solid phase could be vseparated from inner layers and subjected to separate analysis.

25 For comparative purposes, the tabulated data presented below in Table XII illustrate typical results obtained in freezing a small quantity of solids out of a charge consisting of normal grade electrolytic aluminum:

Table XII Speed of Rotation: 66 r.p.m. Retort lnelined: 4 from horizontal. Percent Solid Frozen: 15.7

, Z6 By the instantaneous K-factor is meant the ratio of percent iron content in the very last layer of solid deposited, to the percent iron content of the liquid in equilibrium with that deposited solid. The experimental value as ob- Test No. Cu Fe Si Mn Zn Ga Ti V Zr Cr Total Al 001 002 004 002 002 002 006 006 002 005 99. 966 000 001 O02 001 001 001 023 019 004 007 99. 939 238 (Liquid) 84.3 001 003 006 002 002 002 002 004 002 005 99. 969

K-factor Indef. .33 33 50 50 .50 11. 5 4. 75 2.0 1 4 With reference to the foregoing data, it may be observed that the K-factors obtained for the elements iron, silicon and gallium are not as good as might be expected for initial deposits, but a generally high concentration in the solid obtains for B-elernents.

In Table XIII, there are listed the results of test runs in which portions of the deposited solid were differentially melted to achieve peeling-off of successive layers to determine the nature in which a combined deposit is actually built up during the freeze retining process.

Table XIII Speed of Rotation: 66 r.p.m.

Retort Inclined: 12.5 from horizontal. Metal: Composite.

tained only approximates the thin final layer, but it is an approximation of considerable interest. It may be noted that the theoretical curve for the instantaneous K-:factor of 0.10 generally agrees with the experimental curve, shown by circled coordinates in FIG. 5, both for the lower curve (solid) as well as for the expected content of iron in the liquid fraction which is shown in the upper half of FIG. 5. It is to be noted, however, that after 76.4% solids have been frozen out of the total, the last deposit Test No. 261 K-factor Percent Weight (lbs.) Percent Fe Frozen of Layer Total No. A

Solid Solid Indiv. Avg. Liq. (by Cum. Liq. (by Cum.

Frac- Solid Frae- Solid tions) tions) 23. 00 None None 133 1 17. 56 (5. 44) 5. 44 17 012) .O12 (.071) .071 2 14. 87 (2. 69) 8. 13 20 008) 011. 040) 055 3 11.18 (3. 69) 11.82 .27 (.030) .017 111) 063 4 7. 55 63) 15. 45 .36 042) O23 117) 064 5 5. 3G (2. 19) 17. 64 45 136) 037 301) O82 It may be noted that the foregoing test represented a good average performance, and, as such, it may be considered truly representative of the best previous experimental runs. The first deposited layer of solid in this test demonstrates only a slight tendency towards reduced purity as compared with the second deposited layer. The following deposited solids increase regularly in iron'con- Vtent to the last layer which is extremely high in contained iron. This illustrates the fact that, in small scale runs wherein only a few pounds of liquid metal remain at the end of the run, there will be insuicient physical coverage for the liquid at that point to be in true equilibrium contact with the depositing solid. Accordingly, errors will be introduced by indiscriminate freezing on the outer fringes of the deposited solid as the liquid pool recedes,

. and also because the rate of freezing will increase greatly towards the end.

From a comparison of the results obtained in Test No. 238 and Test No. 261, it is clear that the initial deposited solid layer in the latter test, although good, was not as high grade as might be expected. This deviation demonstrated clearly that aluminum metal forms the best surface for receiving newly depositing solids, as compared to the refractory lining. It seems also that the first deposit on the refractory lining has the character of being a forced deposit, Vand that thereafter it undergoes a further puriiication by exchange.

The curve of increasing impurities in the depositing solid phase of Test No. 261 is plotted for the instantaneous K-factor of 0.10 for iron in FIG. 5 of the drawings.

contains substantially more iron than might be expected from the theoretical plot.

The average K-factor, which has been used herein in all of the tabulated experimental data, is arrived at by melting the total deposited solid phase, and thereafter removing an average sample for analysis, i.e., without regard to the concentration of elements contained in the successive solid fractions. In essence, the average K-factor is most important for a practical evaluation of the process of the invention, whereas the instantaneous K-factor is of greater theoretical and design value. The deposition curve for the average solid is shown in FIG. 5 as appearing below the curve for the instantaneous solid, which is in accord with theory.

The foregoing expedient involving use of an initial layer of aluminum to commence the depositing solid phase has been embodied in the split freeze-refining technique illustrated in FIG. 8 of the drawings. That is to say, the liquid phase is only partially frozen out in the initial stage, and is then passed on in diminishing quantity to a subsequent retort, and so on, until the desired concentration has been achieved. For a continuous operation involving many retorts, the return liquids may be disposed of advantageously by cycling to a preceding stage of another retort, it being understood that all retorts are spaced in time sequence, in proportion to the total cycle time. A residual liquid may be added to the start of la preceding operation, or halfway through the refining cycle, or even near the end of a run, with good results, but the preferred introduction would be at the last charging of a preceding retort, since a larger percentage of metal could thereby be removed from the liquid phase without disrupting the relative purity of the finally deposited solid layer from the previous charge. Return liquids, substantially free of B-elements, may also be upgraded advantageously in' special retorts.

To gain a more complete insight over conditions prevailing within the retort during freeze refining, several additional tests were conducted in conjunction with close observation of prevailingtemperatures. In -those tests conducted in the absence of internal heating, the temperat-ure of the liquid pool dropped off quickly to the melting point of the metal, while crystals were formed in the liquid and the over-all mass gradually thickened. The temperature in the middle of the retort directly beneath the ceiling was about 600 C., while the liquid phase remained constant at about 657? C. throughout the run. The latter temperature was later confirmed to be the melting point of this metal.

In still other tests utilizing the same metal, but with heat applied to the retort in the preferential manner, the temperature of the liquid slowly dropped to the melting point of 657 C., but no crystals formed in the liquid. The temperature within the retort directly beneath the ceiling, now registered from l to 35 C. above the melting point of the liquid phase, which was again established to be 657 C. The temperature immediately above the Kgas fiame deflector registered about 800 C.

In the foregoing tests, the rate of freezing was established to be about 17.5 pounds of solid per hour with internal heating, and about 35 pounds per hour with no internal heating. Most of the better K-factors obtained in these tests represented operations c-onducted with a freezing rate of between -20 pounds per hour. For example, Table XIV below illustrates the comparative refining action for a metal frozen at the rate of only 7.6 pounds per hour, versus the same metal frozen at the rate of 26 pounds per hour:

partly immersed deposited solid moves at a speed of, say between 2-5 feet per second through a large liquid reservoir of metal, with the depositing cycle being broken by a half cycle of cooling and firming-up of the irnmediately preceding deposit. .Again, in zone refining the solid phase remains stationary and the liquid phase is moved, whereas in the present process the solid phase is moved through the liquid phasel in a semi-continuous fashion.

It follows from the results `given that extremely close separations can be effected by the process of the invention, and one may, under different circumstances, arrange to provide reduced or accelerated freezing rates, depending, for example, -on whether maximum separation or maximum production is desired. Essentially, the important Variables in the process -of the invention include the thermal insulating quality ofthe retort lining, the input, quantum and placement of the internal heating, the speed of rotation of the retort, the maintenance of effective longitudinal and transverser agitation'of the liquid phase, and the degree of inclination of the retort from its normal horizontal axis. In this connection, it should also be noted -thatthe deposition surface of the refractory lining of the retort should be amenable to adhesion or anchoringof the initial metallic film. As.

pointed out hereinbefore, it is most advantageous to establish a preliminary coating of aluminum on the refractory. Usually, most refractory surfaces demonstrate sufficient wetting orA penetration by aluminum metal in the liquid phase to serve as the deposition surface, per se. With surfaces of carborundum,` graphite, and the like, which are not readily wetted by liquid aluminum, is found that the use of a small amount of a salt flux aids appreciably in the wetting and adhesion of the initial liquid metallic coating. A combination of a heat-conductive inner frontal refractory of carborundu-rn with the rest of the lining being formed of a less conductive Table XIV Type A impurities Type B Impurities Freezing Test Sample Rate, No. lbs/hour Cu Fe Si Ga Ti V Zr 150--. sq1id .000 .001 .002 .001 .010 .014 .00s 7.6

L1qu1d .005 .016 .01s .004y .000 .000 .001

Kfactor Indef. .062 .l1 .25 .l0 .14 6.0

245.-. sq1id .000 .001 .002 .000 .001 .001 26.1

L1qu1d .000 .005 .017 .001 .000 .000

K-factor-. Indef. .2() .12 Indef. 1 1

It will be noted that the differences in K-factors for layer of alundum, is found to be a preferred arrangethe foregoing tests is relatively small for silicon but v ment for use vin the refining of aluminum. v appreciable for iron, and that the lower rate of freezing 5U The preceding discussions have been directed to procshows the more pronounced separatory action. At a essing techniques in which deposition of a purified solid normal freezing rate of about 20 pounds per hour, figurphase is effected with elimination, or eventual re-cycling ing approximately 2 square feet of cylindrical wall area of residual liquid fractions of varied purities. In anof the retort, there is deposited about l0pounds per other version of the process which has been used to hour per square foot of wall surface. This represents advantage, it is possible to effect selective re-melting of a deposit of about 3A of an inch thickness on the retort the last portion of a previously deposited solid phase, wall after one hour of freezing. In other words, in this and thereafter permit a new exchange between the molten case the solid deposit grows at an average rate of apand solid phases. This. is of particular usefulness in proximately 1/80 of an inch per minute or for each revoobtaining sharper separation of the Type B impurities, lution of the retort the incremental increase in thickness 60 i.e., those impurities tending to raise the normal melting will be 1/5300 of an inch. This is a relatively slow growth point of the aluminum metal. as compared -to zone refining, for example, where the Another useful modification of the basic process of the solid phase `grows at a rate of, for example, 1/s of an invention involves adjustment of a previously deposited inch per minute, or about ten times .the rate of the freeze solid phase, with respect to Type A impurities, to control refining technique of the invention for the example cited. the nally deposited product. In essence, this amounts to The difference between zone refining and freeze rea sort of half-step freezing cycle, and is logically based fining as practiced in accordance with the inventors inupon the deposition curves as shown in FIGS. 5 and 6, vention, is essentially that in some refining a narrow wherein it will be noted that the initial deposits of the liquid pool of metal traverses a long, solid ingot at solid phase are contaminated relatively slowly, 'but after la the rate indicated, whereas in the present process the approximately one half of the total available aluminum has been deposited, the concentration of impurities in the solid phase increases more rapidly, since the liquid in equilibrium with the solid phase at this point is substantially more -contaminated with respect to impurities of the Type A category.

To illustrate the foregoingl technique, another series of tests was conducted, in which a large amount of solid was deposited initially and the highly impure final liquid phase removed from the retort. Thereafter, the gas burner was re-inserted in the retort and a portion of the deposited solids melted down to form a fairly large liquid phase, while the retort was rotated at a speed of 66 r.p.m. The gas burner was then equipped with the metal defiector and operations commenced according to normal freezerefining practice. Heat was supplied to the liquid phase via the frontal refractory lining for Aa period of twenty minutes to effect slow deposition of metal therefrom. At this time, the small residual liquid was discharged, and is called Solid-A below. Then the remainder of the solid was melted out, and this is referred to las Solid-B in the table below. The results of this test are tabulated in Table XV:

Table XV (TEST NO. 264) Speed of Rotation: 66 r.p.m. Retort Inelined: 12.5 from Horizontal. Charge: Composite.

With reference to the foregoing data, it will be noted that in the remelting and freeze-refining cycle, from which Solid-A was discharged, a new equilibrium condition was obtained between the solid and liquid phases, for which an average K-factor of 0.085 was established. Accordingly, the resultant Solid-B was considerably improved in purity. Considering Solid-B, with a content of only 0.04% Fe as compared with the final residual liquid of 0.67% Fe, one obtains an average K-factor of 0.06 for the run. On the other hand, if one adds Solid-A to the final resid-ual liquid, there is obtained an average content of 0.615% Fe, and the K-factor of-Solid-B over the combined liquid phase then `becomes 0.065, or only slightly higher than that obtained on the basis of Solid-B and the original residual liquid phase.

It will be evident that in re-melting a portion of a previously deposited solid phase, and letting it undergo adjustment to obtain new equilibrium conditions, Aone also counteracts inaccuracies in the original freeze-refining cycle causedby insufficient physical coverage and some indiscriminate freezing-out of liquid on the fringes of the ldeposit as well as too fast freezing towards the end. In

essence, this method of operation provides a means 'for leveling off the impurity content of a deposit, such that the finally deposited solid is reconstituted to conform more closely to the initial higher purity fractions of the overall deposit. The method thereby contributes to a more efficient refining action, requiring less manipulation, and promoting an increased direct output of refined metal from each retort. The method is of particular advantage in situations in which limited retorts are available, or conversely which reduces the over-all number of retorts required in any given operation. The adjustment freezing proceeds rapidly, since conditions are excellent for growth of a new deposit on the previously established high-purity aluminum surface. The temperature of the liquid phase metal during the refining was found to be very nearly 30 the same in 'both freezing operations, close to 659 C., or actually about 1 to 2 C. above the melting point of the liquid metal obtained. The method has also been used advantageously for removing A-impurities from the tail-end of the deposit, followed `by a differential melt-out of a portion of the final solid, but leaving as standing on the wall the initial deposit containingV the bulk of B-elements.

In the processing of scrap aluminum, it was observed that the contents of silicon and copper did not hinder the refining action when present in concentrations sufficiently removed from the eutectic concentrations, except, of course, that the temperature of the liquid phase must be reduced in accordance with the corresponding theoretical values obtained from the appropriate phase diagrams.

A content of up to approximately 10% of magnesium could also be dealt with effectively except that the iron elimination was unsatisfactory. This may be explained by the fact that in such an alloy system, the melting point is reduced to approximately 620 C., which is far below the eutectic temperature for the normal iron-aluminum alloy system (654 C.), and that, accordingly, the liquid solubility of iron in aluminum is very much reduced. In order to effect more efficient removal of iron from a near eutectic liquid concentration of 1.50% Fe,

recovered in earlier refining runs, it was found that settling y stages were quite effective. In particular, by employing as high as 28% magnesium additive in this iron-aluminum alloy system, the liquid solubility of iron is adjusted to about 0.18% Fe and an iron-aluminum compound of composition approximately A13Fe will settle out, thereby promoting effective iron separation. By distilling the magnesium away `by vacuum distillation techniques or the like, such that the resulting metal contains only a few percent magnesium, the liquid may then be treated by the freeze refining technique of the invention with excellent results.

Advantageously, the settling-out of iron-aluminum compounds from the 28% magnesium-aluminum-iron system can be effected in the rotary retort of the invention, under rotational speeds suicient to suspend the total liquid contents on the cylindrical lining, such that the settling action occurs over the relatively short distance to the peripheral wall and at a temperature slightly above the melting point. The depositing compound of composition approximating AlgFe, has a theoretical iron content of 40.7% and a melting point of about l C., which favors close separation after the settling action has taken place. This is done by tilting the retort to a vertical position and deaccelerating the retort to pour off the liquid metal, by then subjecting the poured-off liquid to vacuum distillation, the magnesium additive is readily removed and the residual aluminum metal, which may contain a little magnesium, may be treated by freeze refining to remove iron and magnesium, etc., therefrom. Significantly, the magnesium additive may constitute scrap magnesium, whereas the distilled magnesium will be a high purity product suitable for marketing as such.

An addition of zinc to the metallic aluminum is not good for freeze-refining purposes, in that, the solid solubility of zinc in aluminum is very high, with the result that relatively poor K-factors are obtained. On the other hand, a few percent of zinc in scrap metal to be refined may be tolerated, with larger amounts being removed by an initial distillation operation, to facilitate good recoveries of high-grade aluminum in subsequent applications of the freeze-refining technique of the invention. For example, considering scrap aluminum in which the content of zinc is quite high, the inventor effected removal of all but about 1% of the zinc by preliminary distillation. If the iron content at this point is around 1%, a single stage of freeze-refining will serve to concentrate the iron in the liquid phase to about 1.5% Fe, which is suitable for the addition of magnesium, and subsequent settling out of an iron aluminum compound in the manner described 

10. IN A PROCESS FOR THE FREEZE-REFINING OF MOLTEN ALUMINUM CONFINED WITHIN A PORTION OF A ROTATING CYLINDRICAL RETORT TO EFFECT SEPARATION AND RECOVERY OF: (1) A PURIFIED SOLID ALUMINUM PRODUCT BY THE INCREMENTAL CRYSTALLINE DEPOSITION THEREOF THE RETORT WALL, AND (2) A LESSPURE RESIDUAL LIQUID PHASE, THE IMPROVEMENTS THAT COMPRISE MAINTAINING SAID MOLTEN LIQUID PHASE SUBSTANTIALLY FREE OF SOLID CRYSTALLINE GROWTH, MAINTAINING CRYSTALLIZATION TEMPERATURES AT THE SOLID-LIQUID INTERFACE, AND MAINTAINING THAT PROTION OF THE RETORT WALL OUT OF CONTACT WITH SAID LIQUID PHASE AT A LOWER TEMPERATURE FAVORING THE BUILD-UP AND DEPOSITION THEREON OF SAID CRYSTALLINE SOLID PHASE BY HEATING SAID MOLTEN ALUMINUM BY THE APPLICATION OF CONTROLLED QUANTIES OF THERMAL ENERGY TO A SELECTED PORTION ONLY OF THE RETORT WALL IN CONTACT THEREWITH. 